Most commercially available storage systems generally include those with disk drives (e.g., hard disk drives (HDDs)), those with solid state drives (SSDs) (e.g., flash drives), and those with a combination of the two. Disk drives have the advantage of being lower cost than SSDs. On the other hand, it is typically faster to read data from an SSD than a disk drive. With the advancement of semiconductor technology, SSDs are becoming cheaper to manufacture. Accordingly, in storage systems with a combination of disk drives and SSDs, it is becoming increasingly advantageous to store a larger percentage of data using SSDs. A challenge is how to design a cost effective storage system in which a larger percentage of data is stored using SSDs.